1. Field of the Invention
The invention relates to a recording drum for use in a video recorder, and in particular provides apparatus for controlling lateral tape wander on the surface of a drum of the fully rotatable type.
2. Description Relative to the Prior Art
In a drum-type video recorder, a drum is provided around which a magnetic recording tape is helically wrapped. At least one magnetic transducer head cooperates with the drum, the head tip extending slightly from the drum surface. During operation, the transducer head is rotated concentrically with respect to the cylindrical surface of the drum at a high speed so as to bring the head repeatedly into contact with the tape, the tape being advanced over the drum, commonly at a rate of 1.5 to 7.5 inches per second. The result is that a signal is recorded in a series of slanted lines, or tracks, on the tape and the high head-to-tape speed enables signals of relatively high frequency content to be recorded.
For a drum-type video recorder commercially available today, the drum construction is generally one of two types -- the stationary drum or the split drum. The stationary drum comprises a cylindrical surface which does not rotate; the magnetic recording tape is advanced over the stationary drum surface.
The split drum is more complex: In one of its forms, the split drum comprises an upper rotatable section and a lower stationary section, which together sandwich a high-speed rotary disc carrying the magnetic transducer head. When the recording tape is wrapped around the drum, approximately one-half of the tape area is supported by the rotatable section and one-half is supported by the stationary section. As the tape is advanced, frictional forces between the tape and the stationary section act to oppose the tape motion, thus creating a tension differential between tape on the supply side and the take-up side. In order to eliminate this tape tension differential, the upper drum section is rotated in the direction of the tape travel, and at a speed greater than the tape speed. Thus, frictional forces between the tape and the upper rotating section tend to assist the tape motion. By properly contouring the surfaces of the rotating and stationary sections to match friction coefficients, the opposing and assisting forces can be made to cancel each other, resulting in zero tape tension differential.
Both the split drum and the stationary drum produce considerable amounts of tape-to-drum friction. Although the split drum uses counterbalancing frictional forces to eliminate tape tension differential, it would be preferable to eliminate the tension differential by eliminating friction altogether. A third type of drum known, but not in popular use, comprises a one-piece fully rotatable drum; and substantially achieves this goal. As tape advances over the drum surface, the entire drum rotates, thereby forming an air layer between the tape and drum surface. The air layer acts as an efficient air bearing, resulting in small amounts of "drag" and almost no tape tension differential. In addition, the one-piece drum is frequently less costly to manufacture than the split drum, since the split drum requires the machining of two cylindrical surfaces instead of the single cylindrical surface required by the one-piece drum.
Despite inherent advantages, certain problems have inhibited the popular use of the one-piece fully rotatable drum. During operation, as noted above, substantially reduced amounts of "drag" are obtained by the formation of an air layer between the tape and the drum surface; and it is this air layer which offers little resistance to lateral tape motions, as well. Thus, slight perturbations, such as those caused by edge irregularities on the tape or by mechanical vibration, can cause the tape to "roam" laterally over the drum surface.
Even in small amounts, tape wander cannot be ignored. For example, during playback of "roaming" tape through a television receiver, visual effects will depend upon the severity of the wander. Effects range from the appearance of "snow" in the picture, caused by a decrease in the signal-to-noise ratio when wander is slight, to a complete loss of picture as wander becomes severe. A video recorder can be designed to tolerate low levels of wander by improving its signal-to-noise ratio, but this approach may price the recorder out of the market.
The problem of tape wander on a fully rotatable drum is caused by a lack of edge guidance. On a six-inch diameter drum, a 360.degree. wrap produces over 18 inches of freely floating recording tape wrapped around the drum. And on a drum of this type, the tape cannot readily be provided with an edge guide since the entire drum rotates. This problem of edge guidance is not encountered in stationary drums or split drums. U.S. Pat. Nos. 3,354,269 and 3,661,311 disclose edge guides for use with stationary drums wherein an edge guide member is fastened to the outer surface of the stationary drum. Split drums generally have an edge guide channel machined into the lower non-rotating section, as disclosed in U.S. Pat. No. 3,840,895.
Although for two different types of drum construction, the above-mentioned edge guides all rely, for their successful operation, upon the presence of a non-rotating section of the drum upon which to fasten or machine an edge guide. None of these edge guides, or variations thereof, can even be used with a fully rotatable drum because the entire drum rotates during operation.
Although attempts have been made to reduce tape wander on a fully rotatable drum, a fully satisfactory solution has been elusive. For example, while it is true that increasing tape tension decreases tape wander, such a technique is not without problems, viz., increased tape tension produces increased levels of tape-to-drum friction, thereby resulting in an unwanted tape tension differential; and increased tape tension results in decreased head life.
Another method of reducing tape wander on a fully rotatable drum is proposed in U.S. Pat. No. 3,510,604 which discloses a rotatable drum consisting of a cylindrical section and a frusto-conical section which during operation produces a more even air layer between the tape and the drum surface. While a more even air layer may reduce wander caused by a non-uniform air layer, it does not inhibit wander produced by other sources (mechanical vibration, tape edge irregularities, etc). In addition, a frusto-conical surface presumably is not the easiest surface to machine.